200 HUBERT DANA GOODALE 



tunateh' for this expectation, in other cases this shadowy area 

 may lie on the side opposite to the blastopore or even half way 

 between these points. Sometimes, small shadowy spots appear 

 scattered over the translucent area. These I have already inter- 

 preted as migrating cells. The appearance of a shadowy area 

 opposite the blastopore may be explained perhaps by reference 

 to fig. 33. The j^olk cells at the anterior end of the archenteron, 

 when looked down upon in the living egg, would appear opaque 

 compared to the rest of the dorsal side, which, although increased 

 by the addition of the cells of the roof of the archenteron, is 

 actually very thin. These shadowy areas are then undoubtedly 

 connected with, the process of gastrulation, but their exact rela- 

 tion to the various phases of gastrulation needs farther investi- 

 gation. 



The origin of the roof of the archenteron. I wish now to discuss 

 the sources from which the cells of the roof of the archenteron 

 are derived. From the history of the yolk cells during gastrula- 

 tion, as well as from a study of the yolk granules given on page 

 178 it is evident that the greater part of the roof of the archenteron 

 is formed from the yolk cells. Even the posterior part of the roof 

 is derived from yolk cells, if by such we mean the lower four cells 

 at the eight-cell stage. If iYiej were from animal cells they should 

 have small granules. Moreover, the upper four cells of the eight- 

 cell stage lie above the region of cell migration (see page 224 

 et seq.). I have endeavored by means of stains as described 

 below, to determine the amount of invagination of the cells which 

 lie above the blastopore when first formed, but thus far I have 

 been unable to do so satisfactorily. However, by breaking open 

 a marked egg preserved in the chromic-acetic acid mixture, I 

 find that the stain when applied to the dorsal lip appears to be 

 invaginated to the amount already indicated by the yolk granules 

 (fig. 34, x"). 



The cells dorsal to the blastopore lip when first formed (figs. 

 28, 29) come to form part of the archenteric roof. Do the cells 

 ventral to the lip also come to lie in the roof of the archenteron, 

 or do they form some other part of the archenteron, perhaps 

 the anterior part of the floor? 



